Cosmic Rays and the Hunt for Ultra-High-Energy Photons at the the Pierre Auger Observatory

**Introduction**

* The podcast discusses the search for **diffuse photons** with energies above tens of PeV, using data from the **Pierre Auger Observatory**.

* These photons are produced by interactions between cosmic rays and interstellar matter or background radiation.

* The measurement of a diffuse photon flux can help us understand the distribution of cosmic rays in the Galaxy and probe models of super-heavy dark matter.

**The Pierre Auger Observatory**

* The observatory uses a surface detector (SD) and an underground muon detector (UMD).

* The SD array consists of **water-Cherenkov detectors (WCDs)**, and the UMD uses **buried scintillators**.

* The study focuses on data from a 2 km² area with 19 WCDs and 11 UMD stations.

* The combination of SD and UMD measurements allows for a more accurate analysis of air showers.

**The Search for Photons**

* Primary photons are difficult to distinguish from the background of charged cosmic rays.

* Photon-initiated air showers are mostly electromagnetic, while hadron-initiated showers have more muons.

* The analysis uses a **muon content estimator (Mb)** to discriminate between photon and hadron events.

* The study uses **15 months of data** collected during the construction of the array.

* A photon-equivalent energy scale is developed for comparing events initiated by different primary species.

**Results and Implications**

* No photon candidate events were identified in the data.

* Upper limits on the integral photon flux were set between 13.3 and 13.8 km−2 sr−1 yr−1 above tens of PeV.

* These limits are the only ones based on measurements from the **Southern Hemisphere** in this energy domain.

* The analysis extends the Pierre Auger Observatory photon search program to lower energies.

* The results provide constraints on models of super-heavy dark matter.

* Future data from the observatory is expected to improve the upper limit by a factor of ~20.

**Article Reference:**

* A. Abdul Halim et al., Search for a diffuse flux of photons with energies above tens of PeV at the Pierre Auger Observatory, 2024,

Acknowledements: Podcast prepared with Google/NotebookLM. Illustration credits: Pierre Auger Observatory